Organic electroluminescence device and display device

ABSTRACT

An organic electroluminescence device having an anode structure on the lower surface which is effective for taking out light efficiently from the cathode on the upper surface in which the organic electroluminescence device comprises an anode, a cathode and an organic layer put between both of them. The organic layer contains an organic light emitting layer that emits light by re-combination of holes supplied from the cathode A and electrons supplied from the cathode. The cathode comprises a laminate structure of an electron injecting metal layer, and a transparent conductive layer ultra thin film which is basically light permeable. The anode contains a metal belonging to the group V or group VI of the periodical table to at least a portion in contact with the organic layer and is basically light reflective. The anode metal is selected from chromium, molybdenum, tungsten, tantalum and niobium. The anode metal has a work function of 4.8 eV or lower.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention concerns an organic electroluminescence devicecapable of taking out emission of light on the side of a cathode of thedevice.

[0003] 2. Description of Related Art

[0004] Electroluminescence devices utilizing electroluminescence(hereinafter simply referred to as an EL device) have been noted for theuse of them as light emitting devices in various kinds of displaydevices since they have features of high visibility due to self-emissionand excellent impact resistance being a completely solid devices.

[0005] The EL devices include inorganic EL devices using inorganiccompounds as the light emitting material and organic EL devices usingorganic compounds as the light emitting material. Among them, since theorganic EL devices can be easily reduced in the size with a drivingvoltage lowered remarkably studies intending for their practical usehave been made earnestly as display devices in next generation. Theorganic EL device basically adopts a structure comprising a laminationof anode/light emitting layer/cathode in which a transparent electrodeformed on a substrate using a glass plate or the like (usually adopt).In this case, emission light is taken out on the side of the substrate.

[0006] By the way, it has been attempted to take out emission light fromthe side of the cathode by making the cathode transparent because of thefollowing reasons in recent years. At first, when the anode is madetransparent together with the cathode, a light emission devicetransparent as a whole can be provided. An optional color can be adoptedas the background color for the transparent light emission device, and adisplay which is colorful in a state other than light emission can beobtained, to enhance the decorative property. When black is adopted forthe background color, contrast upon emission is improved. Next, when acolor filter or a color conversion device is used, such component can beplaced on the light emission device. Accordingly, the device can bemanufactured with no particular consideration on these layers. As themerit of this, for example, a substrate temperature can be made higherupon forming the anode thereby enabling to lower the resistance value ofthe anode.

[0007] Since the foregoing advantages can be obtained by making thecathode transparent, it has been attempted to manufacture an organic ELdevice by using a transparent cathode. For example, Japanese PatentLaid-Open Hei 10-162959 disclose an organic EL device in which anorganic layer containing an organic light emitting layer is interposedbetween an anode and a cathode, and the cathode comprises an electroninjecting metal layer and an amorphous transparent conductive layer, andthe electron injecting metal layer is in compact with the organic layer.For illustrating the background of the invention, constitutionsdescribed above will be explained briefly.

[0008] At first, the amorphous transparent conductive layer constitutingthe cathode in the organic EL device is to be explained. Any ofamorphous transparent conductive layers may be used so long as it isamorphous and has transparency and it preferably comprises a specificresistivity of 5×10⁻⁴ Ω·cm or lower for avoiding voltage drop and notuniform light emission attributable to this. Further, In—Zn—O seriesoxide films are preferred for the material. In—Zn—O series oxide layeris a transparent conductive film comprising an amorphous oxidecontaining indium (In) and zinc (Zn) as main cationic elements.

[0009] Then, the electron injecting metal layer is to be explained. Theelectron injecting metal layer is a layer of a metal capable ofsatisfactorily injecting electrons to an organic layer containing alight emitting layer. It is desirable that the light transmittance is50% or higher for obtaining a transparent light emitting device and, forthis purpose, an ultra thin film of about 0.5 to 20 nm thickness isdesirable. The electron injecting metal layer can include, for example,those layers of 1 nm to 20 nm film thickness by using metals having awork function of 3.8 eV or lower (electron injecting metal), forexample, Mg, Ca, Ba, Sr, Li, Yb, Eu, Y and Sc. A constitution providinglight transmittance of 50% or more, particularly, 60% or more ispreferred in this case.

[0010] The organic layer interposed between the anode and the cathodecontains at least a light emitting layer. The organic layer may be alayer only consisting of a light emitting layer, or it may be amulti-layered structure in which a hole injecting/transporting layer orthe like is laminated together with the light emitting layer. In theorganic EL device, the organic layer has (1) a function capable ofinjecting holes by the anode or the hole transport layer and capable ofinjecting electrons from the electron injecting layer, (2) atransporting function of moving the injected charges (electrons andholes) under the effect of an electric field and a light emittingfunction of providing re-combination sites of electrons and holes in theinside of the light emitting layer and providing light emissiontherefrom. The hole injecting/transporting layer is a layer comprising ahole transfer compound, which has a function of transmitting holesinjected from the anode to the light emitting layer, in which more holesare injected to the light emitting layer at a lower electric field byinterposing the hole injecting/transporting layer between the anode andthe light emitting layer. In addition, electrons injected from theelectron injecting layer in the light emitting layer are accumulatednear the boundary in the light emitting layer by the energy barrierpresent at the boundary between the light emitting layer and the holeinjecting/transporting layer thereby improving the emission efficiencyof the EL device and providing an EL device of excellent emittingperformance.

[0011] There is no particular restriction on the anode so long as theanode shows conductivity of the work function of 0.8 eV or higher. Ametal, a transparent conductive film (conductive oxide film) orcombination of them having a work function of 4.8 eV or higher ispreferred. It is not always necessary that the anode is transparent and,for example, a carbon layer of black color may be coated. Suitable metalcan include, for example, Au, Pt, Ni and Pd and suitable conductiveoxide can include, for example, In—Zn—O, In—Sn—O, ZnO—Al and Zn—Sn—O.Further, the laminate can include, for example, a laminate of Au andIn—Zn—O, a laminate of Pt and an In—Zn—O and a laminate of an In—Sn—Oand Pt. Since it may suffice that the boundary with the organic layer inthe cathode has a work function of 4.8 eV or higher, the anode may beformed as a dual layer in which a conductive film of a work function of4.8 eV or lower may be used on the side not in contact with the organiclayer. In this case, a metal such as Al, Ta or W or an alloy such as Alalloy or Ta—W alloy may also be used. Furthermore, a conductive polymerssuch as doped polyaniline or doped polyphenylenevinylene, an amorphoussemiconductor such as α-Si, α-SiC or α-C or crystallite such as a μC-Sior μC-SiC may also be used. Furthermore, Cr₂O₃, Pr₂O₅, NiO, Mn₂O₅ orMnO₂ as black semiconductive oxide may also be used.

[0012] As has been described above, Japanese Patent Laid-Open Hei10-162959 discloses a technique of taking out light from the side of thecathode by forming the cathode with an ultra-thin electron injectingmetal layer and an amorphous transparent conductive layer. However,improvement for the anode is not mentioned. That is, the literaturecontains no descriptions for the anode on the lower side which iseffective for efficiently taking out light from the cathode on the upperside. It merely describes that the use of a metal, a transparentconductive film or a combination of them with conductivity showing awork function of 4.8 eV or higher can be used for the anode. It mentionsto Au, Pt, Ni and Pd as suitable metals. However, such metals can not besaid to have satisfactory adhesion with the organic layer but tend tocause dark spots (non-emission point) or not uniform light emission.Further, fine fabrication technique for the metal has not yet beenestablished and highly fine pattering is difficult.

SUMMARY OF THE INVENTION

[0013] In view of the technical subjects in the prior art describedabove, this invention intends to provide an organic electroluminescencedevice having an anode constitution on the lower side which is effectiveto efficiently take out light from the cathode on the upper side.

[0014] The purpose of the invention can be attained in accordance withthis invention in an organic electroluminescence device comprising ananode, a cathode and an organic layer put between them, in which theorganic layer contains an organic light emitting layer that emits lightby re-combination of holes supplied from the anode and electronssupplied from the cathode, wherein

[0015] the anode contains a metal belonging to the group V or group VIof the periodical table to at least a portion in contact with theorganic layer.

[0016] Preferably, the metal is selected from chromium, molybdenum,tungsten, tantalum and niobium,

[0017] The work function of the metal is 4.8 eV or lower. Further, theanode has a reflectance of 40% or higher.

[0018] The anode is light reflecting while the cathode is lightpermeable and emission light is mainly emitted from the side of thecathode.

[0019] Preferably, the cathode, the organic layer and the anode arelaminated orderly from above to a substrate.

[0020] This invention also includes a display device utilizing theorganic electroluminescence device described above for a pixel.

[0021] That is, a display device according to this invention basicallyhas a constitution in which scanning lines for selecting pixels and datalines for providing luminance information for driving the pixels arearranged in a matrix, each of the pixels comprises an organicelectroluminescence device that emits light in accordance with theamount of current supplied, a first active element controlled by thescanning lines and having a function of writing the luminanceinformation given from the data lines to the pixels and, a second activeelement having a function of controlling the amount of current suppliedto the organic electroluminescence device in accordance with the writtenluminance information, the luminance information is written to each ofthe pixels by applying electric signals in accordance with the luminanceinformation to the data lines in a state where the scanning line isselected, the luminance information written in each of the pixels isheld in each of the pixels even after the scanning line becomes no moreselected, and the organic electroluminescence device in each of thepixels can maintain the light emission at a luminance in accordance withthe luminance information.

[0022] The organic electroluminescence device comprises an anode, acathode and an organic layer put between them and the organic layercontains an organic light emittinq layer that emits light byre-combination of holes supplied from the anode and electrons suppliedfrom the cathode.

[0023] The anode contains a metal belonging to the group V or group VIof the periodical table to at least a portion in contact with theorganic layer.

[0024] The metal is preferably selected from chromium, molybdenum,tungsten, tantalum and niobium.

[0025] Further, the metal has a work function of 4.8 eV or lower.

[0026] Further, the cathode has a reflectance of 40% or higher. Theanode is light reflecting and the cathode is light permeable in whichemission light is mainly emitted from the side of the cathode.

[0027] Each of the pixels is accumulated and formed on the substrate,and the organic electroluminescence device contained in each of thepixels comprises the cathode, the organic layer and the anode laminatedin this order from above to the substrate.

[0028] According to this invention, the anode of the organicelectroluminescence device comprises a metal belonging to the group V orgroup VI of the periodical table. The metal can include high meltingmetals such as chromium, molybdenum, tungsten, tantalum and niobium.Such metals have a work function of 4.8 eV or lower, for example,chromium having 4.5 eV and tungsten having 0.6 eV. Further, thereflectance is 40% or higher.

[0029] Metals having somewhat higher work function of 4.8 eV or higher(Au, Pt, Ni, Pd, etc.) have been adopted for the anode so far in view ofthe requirement of supplying holes. In this invention, metals belongingto the group V or the group VI having lower work function (Cr, Mo, W,Ta, Nb, etc.) are used. It has been confirmed that even the metalsbelonging to the group V or the group VI can effectively supply theholes. Rather, chromium (Cr) and the like have less defects andexcellent fabricability compared with gold (Au) and the like, as well asthey are excellent as a whole as a material for the anode of the organicelectroluminescence device.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0030]FIG. 1 is across sectional view showing a basic constitution of anorganic EL device according to this invention;

[0031]FIG. 2 is a cross sectional view showing a constitution of anorganic EL device according to a reference embodiment;

[0032]FIG. 3 is a step chart showing a method of manufacturing theorganic EL device according to this invention;

[0033]FIG. 4 is a step chart also showing a method of manufacturing theorganic EL device according to this invention;

[0034]FIG. 5 is a step chart also showing a method of manufacturing theorganic EL device according to this invention;

[0035]FIG. 6 is a step chart view also showing a method of manufacturingthe organic EL device according to this invention;

[0036]FIG. 7 is an enlarged plan view showing a light emitting surfaceof an organic EL device;

[0037]FIG. 8 is an enlarged plan view also showing a light emittingsurface of an organic EL device;

[0038]FIG. 9 is an enlarged plan view also showing a light emittingsurface of an organic EL device;

[0039]FIG. 10 is an enlarged plan view also showing a light emittingsurface of an organic EL device;

[0040]FIG. 11 is an equivalent circuit diagram showing one pixel of adisplay device according to this invention;

[0041]FIG. 12 is a block diagram showing an entire constitution of thedisplay device according to this invention; and

[0042]FIG. 13 is a cross sectional view showing a structure of thedisplay device according to this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] Preferred embodiments of this invention are to be explained indetails with reference to the drawings.

[0044]FIG. 1 is across sectional view showing a basic constitution of anorganic electroluminescence device according to this invention. As shownin the drawing, the organic electroluminescence device comprises ananode A, a cathode K and an organic layer 10 put between both of them.The organic layer 10 contains an organic light emitting layer 103 thatemits light by re-combination of holes supplied from the anode A andelectrons supplied from the cathode K. Further, the organic layercontains a hole injection layer 101 and a hole transparent layer 102.The cathode K has a laminate structure of an electron injecting metallayer 11 and a transparent conductive layer 12 each of an extremelyreduced thickness. As the feature, the cathode A contains a metalbelonging to the group V or group VI of the periodical table to at leasta portion in contact with the organic layer 10. The anode metal ispreferably selected from chromium, molybdenum, tantalum and niobium.Further, the anode metal has a work function of 4.8 eV or lower, forexample, chromium having 4.5 eV and tungsten having 0.6 eV. Further, thereflectance of the anode A comprising such metal is 40% or higher. Thatis, the anode is light reflecting and the cathode is light permeable inwhich emission light is mainly emitted from the side of the cathode. Thecathode K, the organic layer 10 and the anode A are laminated in thisorder from above to the substrate 1.

[0045] The anode A may be a single layer of pure metal, as well as alamination layer or an alloy. Basically, it may suffice that the metalbelonging to the group V or group VI of the periodical table iscontained at a portion in contact with the organic layer 10. The anode Acomprises a metal, an alloy or a laminate thereof.

[0046] For example, when a chromium film was formed to a 200 nmthickness as the anode A on the glass substrate 1 and the reflectancethereof was measured, it was 67% at a wavelength of 460 nm. Further, asthe cathode K, an ultra-thin electron injecting metal layer 11comprising an Mg:Ag alloy was formed to a extremely reduced filmthickness of 10 nm and, further, a transparent conductive layer 12 wasformed to a thickness of 200 nm being stacked thereon. When thetransmittance of the laminated cathode K was measured at a wavelength460 nm, it was 53%. When 8 volt voltage was applied between theanode-cathode of the organic EL device formed as shown in the figure byusing the anode A and the cathode K described above, a current of 20mA/cm² was observed, and emission luminance of 900 cd/m² was observedfrom the side of the cathode K. A considerable amount of emission lightdirected toward the anode A is reflected and proceeds oppositely and isthen emitted from the cathode A. Favorable carrier injectioncharacteristics and light emission characteristics could be confirmed.Further, no dark spots were observed on the light emitting surface.

[0047] As a comparative example, an organic EL device shown in FIG. 2was prepared. The constitution was basically identical with thatillustrated in FIG. 1, in which corresponding portions carrycorresponding reference numerals. An organic EL device was manufacturedin the same manner as that in FIG. 1 excepting that the anode A was madeof a transparent conductive film ITO. When 8 volt of voltage was appliedbetween anode-cathode of the thus manufactured organic EL device, acurrent of 23 mA/cm² was observed but the emission luminance from thecathode K was as low as 250 cd/m² compared with the organic EL deviceshown in FIG. 1. This indicates that the emission light transmitted inthe direction of the anode A was scarcely reflected but emitted from theglass substrate 1. As apparent from the result of comparison describedabove, since the organic EL device manufactured according to thisinvention can effectively take out emission light generated in theorganic light emitting layer 103 from the upper surface, satisfactorylight emission on the upper surface can be obtained.

[0048] Now, a method of manufacturing an organic EL device according tothis invention is to be explained in details with reference to FIG. 3 toFIG. 6. In this embodiment, chromium was used as the anode comprisingthe metal. chromium has a work function of 4.5 eV. As shown in FIG. 3,chromium (Cr) was formed to a film thickness of 200 nm on a glasssubstrate 1 by DC sputtering. Sputtering was conducted using argon (Ar)as a sputtering gas under a pressure of 0.2 Pa and with DC power of 300W. It was patterned to a predetermined shape by using ordinarylithography. It was fabricated by using ETC-1 as an etching solution(manufactured by Sanyo Kasei Industry Co.). An anode A of apredetermined shape can be obtained. Chromium can be fabricated at ahigh accuracy and good reproducibility by the etching solution. When ahigher fabrication accuracy is required, it may be fabricated by dryetching. As the etching gas, a gas mixture of chlorine (Cl₂) and oxygen(O₂) can be used. Particularly, fabrication can be applied at a highaccuracy and the shape of the etched surface can be controlled by usingreactive ion etching (RIE). The device can be fabricated into a taperedshape by etching under predetermined conditions thereby enabling toreduce cathode-anode short circuit.

[0049] As shown in FIG. 4, an insulative layer 15 is formed on thesubstrate 1 on which chromium was fabricated into a predeterminedpattern. There is no particular restriction on the material used for theinsulative layer 15 and silicon dioxide (SiO₂) was used in thisembodiment. SiO₂ was formed to a film of 200 nm thickness by sputtering.There is no particular restriction for the method of forming the film.SiO₂ was fabricated so as to form an opening on chromium by usingordinary lithography. A mixed solution of hydrofluoric acid and ammoniumfluoride can be used for the etching of SiO₂. Further, fabrication bydry etching is also possible. The opening forms a light emitting portionof the organic EL device. Although the insulative layer 15 is notessential in this invention, it is desirably disposed in order toprevent anode-cathode short circuit.

[0050] Successively, as shown in FIG. 5, the glass substrate 1 formedwith chromium and SiO₂ was placed in a vacuum vapor deposition apparatusand an organic layer 10 and a metal layer 11 of the cathode K wereformed by vapor deposition. In this organic layer 10,4,4′,4″-tris(3-methylphenylamino) triphenylamine (MTDATA) was used as ahole injecting layer 101, bis(N-naphthyl)-N-phenylbendizine (α-NPD) wasused as a hole transporting layer 102 and 8-quinolinol aluminum complex(Alq) was used as the light emitting layer 103.

[0051] An alloy of magnesium and silver (Mg:Ag) was used as the metallayer 11 of the cathode K. Each of the materials for the organic layer10 was filled in a boat for ohmic resistance each by 0.2 g and thenattached to a predetermined electrode in the vacuum vapor depositionapparatus. For the metal layer 11, magnesium was filled by 0.1 g andsilver was filled by 0.4 g to a boat and attached to a predeterminedelectrode of the vacuum vapor deposition apparatus.

[0052] After reducing the pressure in the vacuum chamber to 1.0 ×10⁻⁴Pa, voltage is applied to each of the boats and heated gradually toconduct vapor deposition. In vapor deposition, the organic layer 10 andthe metal layer 11 comprising Mg:Ag were vapor deposited only to apredetermined portion by using a metal mask. The predetermined portionis a portion in which chromium is exposed on the substrate 1. Since itis difficult to conduct vapor deposition at high accuracy only to theportion where the chromium is exposed, a vapor deposition mask wasdesigned so as to cover the entire portion where the chromium wasexposed (so as to extend to the edge of the insulative layer 15).

[0053] At first, there were deposited MTDATA by 30 nm as the holeinjecting layer 101, α-NPD by 20 nm as the hole transporting layer 102and Alq by 50 nm as the light emitting layer 103. Further, Mg:Ag wasformed as the metal layer 11 of the cathode K on the organic layer 10 byconducting co-vapor deposition of magnesium and silver. The film formingrate was set to 9:1 between magnesium and silver. The thickness for thefilm of Mg:Ag is made to 10 nm.

[0054] Finally, as shown in FIG. 6, the device was transferred inanother vacuum chamber, and a transparent conductive layer 12 was formedthrough an identical mask. DC sputtering was used for the filmformation. In this embodiment, an In—Zn—O series transparent conductivefilm showing good conductivity in film formation at a room temperaturewas used as the transparent conductive layer 12. The film was formed bysputtering under the conditions of using a gas mixture of argon andoxygen (volume ratio Ar:O₂=1000:5) and under a pressure of 0.3 Pa with aDC power of 40 W. The film was formed to a thickness of 200 nm.

[0055] As the material for the anode A, tungsten may also be used inaddition to chromium. In this case, tungsten (W) was formed to athickness of film thickness of 200 nm on the glass substrate by DCsputtering. Sputtering was conducted using argon (Ar) under a pressureof 0.2 Pa with a DC power of 300 W. Then, patterning was applied by dryetching. CF₄ or SF₆ can be used as the etching gas. Particularly,fabrication at high accuracy is possible and the shape of the etchingcircuit can be controlled by using the reactive ion etching (RIE). Whenetching is applied under a predetermined condition, the device can befabricated into a tapered shape thereby enabling to decreasecathode-anode short circuit. Succeeding steps were identical with thosefor chromium.

[0056] Then, the appearance and the characteristics of the organic ELdevice are explained with reference to FIG. 7 to FIG. 10. FIG. 7 is aphotograph taken for the light emitting surface of the embodiment usingchromium (Cr) for the anode A. The light emitting surface is of 2 mmsquare and dark spots are observed slightly (non-light emission point).FIG. 8 is a photograph taken for the light emitting surface of theembodiment using tungsten (W) for the anode A. The light emittingsurface is of 2 mm square and dark spots are also observed slightly(non-light emission point) FIG. 9 is a photograph taken for the lightemitting surface of a reference example by using ITO as the anode A inwhich considerable amounts of dark spots (non-emission point) areobserved. FIG. 10 is a photograph taken for the light emitting surfaceof the reference example using gold (A8) as the anode A in which a greatamount of dark spots (non-light emission point) are observed. This isattributable to poor adhesion between gold and the organic layer.

[0057] Finally, a display device using the organic EL device of thisinvention for the pixel is to be explained. Generally, in active matrixtype display devices, images are displayed by arranging a plurality ofpixels in a matrix and controlling the intensity of light on everypixels in accordance with given luminance information. In a case whereliquid crystals are used as an electro-optical substance, thetransmittance of the pixel changes in accordance with the voltagewritten in each of the pixels. Also in the active matrix type displaydevice using the organic electroluminescence material as theelectro-optical substance, the basic operation is identical with thatusing the liquid crystals. However, different from the liquid crystaldisplay, the organic EL display is a self-emission type having alightemitting device in each of the pixels, and provides merits such ashigher visibility of images compared with the liquid crystal display,not requiring back light and high response speed. The luminance ofindividual light emitting devices is controlled by the amount ofcurrent. That is, the organic EL display is greatly different from theliquid crystal display in that the light emitting device is of a currentdriven type or a current control type.

[0058] In the same manner as in the liquid crystal display, the organicEL display can also adopt a simple matrix system and an active matrixsystem as the driving system. Since the former has a simple structurebut it is large in the scale and difficult to attain display at highfineness, development has been conducted vigorously for the activematrix system. In the active matrix system, current flowing to theorganic EL device disposed to each of the pixel is controlled by anactive element disposed inside of the pixel (generally a thin filmtransistor as a kind of insulated gate type field effect transistors,(hereinafter sometimes referred to as TFT)). For the organic EL displayof the active matrix system, an equivalent circuit for one pixel isshown in FIG. 11. The pixel PXL comprises an organic EL device OLED, athin film transistor TFT1 as a first active element, a thin filmtransistor TFT2 as a second active element and a holding capacitance Cs.Since the organic EL device often has a rectifying property, it maysometimes be called as OLED (Organic Light Emitting Diode) and it isdenoted by a symbol for a diode. In the illustrated embodiment, thesource S of TFT2 is defined as a reference potential (ground potential)and the cathode K of OLED is connected to Vdd (power source potential),while the anode A of OLED is connected to a drain D of TFT2. On theother hand, the gate G of TFT1 is connected to a scanning line X, thesource S of TFT1 is connected to data line Y, and the drain D thereof isconnected to the holding capacitance Cs and the gate G of TFT2.

[0059] For operating the PXL, when the scanning line X is at first whenselected, and a data potential Vdata indicative of the luminanceinformation is applied to the data line Y, TFT1 becomes conductive andthe holding capacitance Cs is charged or discharged to align the gatepotential of TFT2 with the data potential Data. When the scanning line Xis put to non-selected state, TFT1 turns off while TFT2 is electricallyisolated from the data line Y, and the gate potential of TFT2 is heldstably by the holding capacitance Cs. The current flowing through TFT2to the organic EL device OLED takes a value in accordance withgate/source voltage Vgs of TFT2, and OLED emits light continuously at aluminance in accordance with the amount of current supplied from TFT2.

[0060] As has been described above, in the circuit structure of thepixel PXL shown in FIG. 11, when Vdata is once written, OLED emits lightcontinuously at a constant luminance during one frame till the nextrewriting. When such pixels PXL are arranged in plurality in a matrix asshown in FIG. 12, an active matrix type display device can beconstituted.

[0061] As shown in FIG. 12, in this display device, scanning lines X1 toXN for selecting the pixels PXL and data lines Y giving the luminanceinformation (data potential Vdata) for driving the pixels PXL arearranged in a matrix. The scanning lines X1 to XN are connected to ascanning line driving circuit 21, while the data lines Y are connectedto a data line driving circuit 22. Desirable images can be displayed byrepetitive writing of Vdata from the data lines Y by the data linedriving circuit 22. In the simple matrix type display device, the lightemitting device contained in each of the pixels PXL emits light only forthe selected instance whereas, in the active matrix type display deviceshown in FIG. 12, since the organic EL device of each of the pixels PXLemits light continuously also after the completion of writing, it isadvantageous, particularly, in a large scale and highly fine display inthat the peak luminance (peak current) of the organic EL device can belowered compared with the simple matrix type.

[0062]FIG. 13 schematically shows a cross sectional structure of thepixel PXL shown in FIG. 11. For easy illustration, only OLED and TFT2are shown. OLED comprises an anode A, an organic layer 10 and a cathodeK stacked successively in this order. The anode A is isolated on everypixel and, in accordance with this invention, the anode A comprises forexample chromium and is basically light reflective. The cathodes K areconnected in common between the pixels and each comprises, for example,a laminate structure of a metal layer 11 and a transparent conductivelayer 12 and which is basically light permeable. When a forward voltage(about 10 V) is applied between anode A and cathode K of OLED havingsuch a constitution, injection of carriers such as electrons or holes istaken place and light emission is observed. It is considered that theoperation of OLED is light emission by exciting units formed with holesinjected from the anode A and electrons injected from the cathode K.

[0063] On the other hand, TFT2 comprises a gate electrode 2 formed on asubstrate 1, for example, made of glass, a gate insulative film 3superposed on the upper surface thereof and a semiconductor thin film 4stacked by way of the gate insulative film 3 above the gate electrode 2.The semiconductor thin film 4 comprises, for example, a polycrystalsilicon thin film. TFT2 comprises a source S, a channel Ch and a drain Dforming a channel for the current supplied to OLED. The channel Ch issituated just above the gate electrode 2. TFT2 of the bottom gatestructure is covered with an interlayer insulative film 5, on which areformed a source electrode 6 and a drain electrode 7. The OLED describedabove is formed as a film by way of another interlayer insulative film 9thereon.

[0064] As has been explained above according to this invention, lightemitted from the light emitting layer can be taken out efficiently fromthe upper electrode side as the cathode. By using a metal having ahigher reflectance than that of the transparent conductive film as theanode, light conducted to the anode is reflected and taken out of theupper electrode side. Further, in this invention, a satisfactory lightemitting efficiency can be obtained. It can provide a substantiallyidentical hole injecting efficiency substantially identical with thatusing the transparent conductive film (for example, ITO) according tothis invention.

[0065] Further, it forms less dark spots (non-light emission point)observed upon light emission. In addition, the anode can be patterned athigh accuracy. A display at high fineness can be manufactured easily.Furthermore, the structure and the process are simple. In a case offorming the anode with ITO as in the prior art, a reflection layer suchas a metal maybe disposed therebelow but the structure and the processare more complicated compared with this invention. Further, since thelight can be taken out efficiently from the side of the upper surfaceelectrode, an organic EL device of large opening ration can bemanufactured on a glass substrate formed with TFT. In a case of takingout light from the lower electrode, since TFT does not transmit lighttherethrough, only several % of opening ratio can be obtained.Accordingly, it is possible to manufacture an active matrix systemdisplay of high performance by using the organic EL device according tothis invention.

What is claimed is:
 1. An electroluminescence device comprising: ananode, a cathode and an organic light emitting layer put between theanode and the cathode in which the anode contains a metal belonging tothe group V or the group VI of the periodical table at least to aportion in contact with the organic light emitting layer.
 2. Anelectroluminescence device as claimed in claim 1, wherein the metalincludes chromium, molybdenum, tungsten, tantalum or niobium.
 3. Anelectroluminescence device as claimed in claim 1, wherein the workfunction of the metal is 4.8 eV or lower.
 4. An electroluminescencedevice as claimed in claim 1, wherein the anode has a reflectance of 40%or higher.
 5. An electroluminescence device as claimed in claim 1,wherein emission light from the organic light emitting layer is emittedmainly from the side of the cathode.
 6. An electroluminescence device asclaimed in claim 1, wherein the anode comprises an alloy.
 7. Anelectroluminescence device as claimed in claim 1, wherein the organiclight emitting layer has a hole transporting layer for transportingholes injected from the anode.
 8. An electroluminescence device asclaimed in claim 1, wherein the cathode comprises a layer consisting ofa metal and a transparent material.
 9. An electroluminescence device asclaimed in claim 1, wherein the cathode comprises MgAg.
 10. Anelectroluminescence device comprising the following constitutions: asubstrate, an anode formed on the substrate, an organic light emittinglayer formed on the anode and a cathode formed on an organic lightemitting layer, in which the anode contains a metal belonging to thegroup V or group VI of the periodical table to at least a portion incontact with the organic light emission device.
 11. Anelectroluminescence device as claimed in claim 10, wherein the metal ischromium, molybdenum, tungsten, tantalum or niobium.
 12. Anelectroluminescence device as claimed in claim 10, wherein the metal hasa work function of 4.8 eV or lower.
 13. An electroluminescence device asclaimed in claim 10, wherein the anode has a reflectance of 40% orhigher.
 14. An electroluminescence device as claimed in claim 10,wherein emission light from the organic light emitting layer is mainlyemitted from the side of the cathode.
 15. An electroluminescence deviceas claimed in claim 10, wherein the anode comprises an alloy.
 16. Anelectroluminescence device as claimed in claim 10, wherein the organiclight emitting layer has a hole transporting layer for transportingholes injected from the anode.
 17. An electroluminescence device asclaimed in claim 10, wherein the cathode comprises a layer composed of ametal and a transparent material.
 18. An electroluminescence device asclaimed in claim 10, wherein the cathode comprises MgAg.
 19. Anelectroluminescence device comprising: scanning lines for selectingpixels, data lines provided with luminance information for drivingpixels, a first transistor connected at a control terminal with thescanning lines, a second transistor connected at a control terminal withthe first transistor, and a light emitting device connected with thesecond transistor in which the light emitting device at least has anorganic light emitting layer, a first electrode providing holes to theorganic light emitting layer and a second electrode providing electronsto the organic light emitting layer, and the first electrode contains ametal belonging to the group V or group VI of the periodical table to atleast a portion in contact with the organic light emitting layer.
 20. Anelectroluminescence device as claimed in claim 19, wherein the firsttransistor and the second transistor are field effect transistors and acapacitance is connected with the control terminal of the second controlterminal.
 21. An electroluminescence device as claimed in claim 19,wherein the scanning lines and the data lines cross substantiallyvertical to each other.
 22. An electroluminescence device as claimed inclaim 19, wherein the metal is chromium, molybdenum, tungsten, tantalumor niobium.
 23. An electroluminescence device as claimed in claim 19,wherein the metal has a work function of 4.8 eV or lower.
 24. Anelectroluminescence device as claimed in claim 19, wherein the firstelectrode has a reflectance of 40% or higher.
 25. An electroluminescencedevice as claimed in claim 19, wherein light emission from the organiclight emitting layer is emitted from the side of the second electrodemainly.
 26. An electroluminescence device as claimed in claim 19,wherein the first electrode comprises an alloy.
 27. Anelectroluminescence device as claimed in claim 19, wherein the organiclight remitting layer has a hole transporting layer for transportingholes injected from the first electrode.
 28. An electroluminescencedevice as claimed in claim 19, wherein the second electrode isconstituted with a layer comprising a metal and a transparent material.29. An electroluminescence device as claimed in claim 19, wherein thesecond electrode comprises MgAg.
 30. An active matrix typeelectroluminescence device comprising: scanning lines for selectingpixels, data lines provided with luminance information for driving thepixels, a first transistor connected at a control terminal with thescanning lines, a second transistor connected at a control terminal withthe first transistor, and a light emitting device connected with thesecond transistor in which the light emitting device at least has anorganic light emitting layer, a first electrode providing holes to theorganic light emitting layer and a second electrode providing electronsto the organic light emitting layer, and the first electrode contains ametal belonging to the group V or group VI of the periodical table to atleast a portion in contact with the organic light emitting layer.
 31. Anactive matrix type electroluminescence device as claimed in claim 30,wherein the first transistor and the second transistor are field effecttransistors and connected at the second control terminals with thecapacitor.
 32. An active matrix type electroluminescence device asclaimed in claim 30, wherein the scanning lines and the data lines crosssubstantially vertically to each other.
 33. An active matrix typeelectroluminescence device as claimed in claim 30, wherein the metal ischromium, molybdenum, tungsten, tantalum or niobium.
 34. An activematrix type electroluminescence device as claimed in claim 30, whereinthe metal has a work function of 4.8 eV or lower.
 35. An active matrixtype electroluminescence device as claimed in claim 30, wherein thefirst electrode has a reflectance of 40% or higher.
 36. An active matrixtype electroluminescence device as claimed in claim 30, wherein lightemission from the organic light emitting layer is emitted mainly fromthe side of the second electrode
 37. An active matrix typeelectroluminescence device as claimed in claim 30, wherein the firstelectrode comprises an alloy.
 38. An active matrix typeelectroluminescence device as claimed in claim 30, wherein the organiclight remitting layer has a hole transporting layer for transportingholes injected from the first electrode.
 39. An active matrix typeelectroluminescence device as claimed in claim 30, wherein the secondelectrode is constituted with a layer comprising a metal and atransparent material.
 40. An active matrix type electroluminescencedevice as claimed in claim 30, wherein the second electrode comprisesMgAg.
 41. A display device comprising: scanning lines for selectingpixels, and data lines disposed substantially vertically relative to thescanning lines and provided with luminance information for driving thepixels in which the pixel at least comprises an organicelectroluminescence device having an anode containing a metal belongingto the group V or group VI of the periodical table to a portion incontact with the organic light emitting layer, and a cathode disposed ata position opposing to the anode, a first active element controlled bythe scanning lines and having a function of intaking luminanceinformation provided from the data lines and a second active elementhaving a function of controlling the current supplied to the organicelectroluminescence device in accordance with the intaken luminanceinformation, the luminance information is taken into the pixels byapplying electric signals in accordance with the luminance informationto the data lines in a state where the data lines are selected, theluminance information taken in the pixel is maintained to the pixel evenafter the scanning line becomes no more selected, and the organicelectroluminescence device maintains light emission at a luminanceaccording to the luminance information.
 42. A display device as claimedin claim 41, wherein the first transistor and the second transistor arefield effect transistors and a capacitance is connected with the controlterminal with the second control terminal.
 43. A display device asclaimed in claim 41, wherein the metal is chromium, molybdenum,tungsten, tantalum or niobium.
 44. A display device as claimed in claim41, wherein the metal has a work function of 4.8 eV or lower.
 45. Adisplay device as claimed in claim 41, wherein the first electrode has areflectance of 40% or higher.
 46. A display device as claimed in claim41, wherein light emission from the organic light emitting layer isemitted mainly from the side of the cathode.
 47. A display device asclaimed in claim 41, wherein the anode comprises an alloy.
 48. A displaydevice as claimed in claim 41, wherein the organic light remitting layerhas a hole transporting layer for transporting holes injected from thefirst anode.
 49. A display device as claimed in claim 41, wherein theanode is constituted with a layer comprising a metal and a transparentmaterial.
 50. A display device as claimed in claim 41, wherein thesecond electrode comprises MgAg.
 51. A method of manufacturing anelectroluminescence device comprising the following steps: a step offorming a first electrode having a metal belonging to the group V or thegroup VI of the periodical table on a substrate: a step of forming anorganic light emitting layer so as to be in contact with the metal and astep of forming a second electrode on the organic light emitting layer.52. A manufacturing method as claimed in claim 51, wherein the firstelectrode is in a tapered type.
 53. A manufacturing method as claimed inclaim 51, wherein the metal is chromium, molybdenum, tungsten, tantalumor niobium.
 54. A manufacturing method as claimed in claim 51, whereinthe metal has a work function of 4.8 eV or lower.
 55. A manufacturingmethod as claimed in claim 51, wherein the first electrode has areflectance of 40% or higher.
 56. A manufacturing method as claimed inclaim 51, wherein first electrode has a higher reflectance than thesecond electrode.
 57. A manufacturing method as claimed in claim 51,wherein the first electrode comprises an alloy.
 58. A manufacturingmethod as claimed in claim 51, wherein the organic light remitting layerhas a hole transporting layer for transporting holes injected from thefirst electrode.
 59. A manufacturing method as claimed in claim 51,wherein the second electrode is constituted with a layer comprising ametal and a transparent material.
 60. A method of manufacturing anelectroluminescence device comprising the following steps: a step offorming a first electrode having a metal belonging to the group V orgroup VI of the periodical table on a substrate, a step of fabricating afirst electrode, a step of forming an insulative film on the firstelectrode, a step of forming an opening in the insulative film andexposing the metal, a step of forming an organic light emitting layer soas to be in contact with the metal through the opening and a step offorming a second electrode on the organic light emitting layer.
 61. Amanufacturing method as claimed in claim 60, wherein the first electrodeis in a tapered type.
 62. A manufacturing method as claimed in claim 60,wherein the metal is chromium, molybdenum, tungsten, tantalum orniobium.
 63. A manufacturing method as claimed in claim 60, wherein thea metal has work function of 4.8 eV or lower.
 64. A manufacturing methodas claimed in claim 60, wherein the first electrode has a reflectance of40% or higher.
 65. A manufacturing method as claimed in claim 60,wherein the first electrode has a higher reflectance than the secondelectrode.
 66. A manufacturing method as claimed in claim 60, whereinthe first electrode comprises an alloy.
 67. A manufacturing method asclaimed in claim 60, wherein the organic light remitting layer has ahole transporting layer for transporting holes injected from the firstelectrode.
 68. A manufacturing method as claimed in claim 60, whereinthe second electrode is constituted with a layer comprising a metal anda transparent material.
 69. A method of manufacturing anelectroluminescence device comprising the following steps: a step offorming a gate electrode on a substrate, a step of forming a gateinsulative film on the gate electrode, a step of forming a semiconductorlayer on the gate insulative film, a step of forming an insulative filmon the semiconductor layer, a step of forming a first electrode having ametal belonging to the group V or group VI of the periodical table onthe insulative film, a step of forming an organic light emitting layerso as to be in contact with the metal and a step of forming a secondelectrode on the organic light emitting layer.
 70. A manufacturingmethod as claimed in claim 69, wherein the first electrode is in atapered type.
 71. A manufacturing method as claimed in claim 69, whereinthe metal is chromium, molybdenum, tungsten, tantalum or niobium.
 72. Amanufacturing method as claimed in claim 69, wherein the metal has awork function of 4.8 eV or lower.
 73. A manufacturing method as claimedin claim 69, wherein the first electrode has a reflectance of 40% orhigher.
 74. A manufacturing method as claimed in claim 69, wherein thefirst electrode has a higher reflectance than the second electrode. 75.A manufacturing method as claimed in claim 69, wherein the firstelectrode comprises an alloy.
 76. A manufacturing method as claimed inclaim 69, wherein the organic light remitting layer has a holetransporting layer for transporting holes injected from the firstelectrode.
 77. A manufacturing method as claimed in claim 69, whereinthe second electrode is constituted with a layer comprising a metal anda transparent material.
 78. A manufacturing method as claimed in claim69, wherein the substrate comprises glass and the gate insulative filmhas a thickness less than that of the insulative film described above.